As a system for realizing a public line network using an optical fiber, an access system optical communication system of point-to-multipoint called PON (Passive Optical Network) system has been widely used.
The PON system includes one OLT (Optical Line Terminal), which is a station-side device, and a plurality of ONUs (Optical Network Units), which are subscriber terminal devices, connected to the OLT via an optical star coupler. The PON system has advantages that economization of operation costs can be expected because the OLT and the most part of optical fibers, which are transmission lines, can be shared among a large number of ONUs, the optical star coupler, which is a passive component, does not require power supply and is easily set outdoors, and the reliability is high. Therefore, the PON system has been actively introduced in recent years as a trump card for realizing a broadband network.
For example, in a 10G-EPON (10 Gigabit-Ethernet (registered trademark) Passive Optical Network) which is capable of performing communication at transmission speed of 10 Gbit/s standardized by IEEE (Institute of Electrical and Electronic Engineers) 802.3av, a broadcast communication system by a 1.58 μm band is used in downward communication from the OLT to the ONUs. The ONUs divide transmission speed using WDM (Wavelength Division Multiplexing) filters for performing wavelength division multiplexing and extract only data addressed to the own stations in time slots allocated to the ONUs. On the other hand, in upward communication from the ONUs to the OLT, an optical wavelength 1.27 μm band is used and a time division multiplex communication system for controlling transmission timing is used to prevent data from the ONUs from colliding with one another.
In the upward communication of the PON system explained above, a light receiving unit of the OLT receives burst light signals. In the OLT, by detecting the light reception level of each of the burst light signals, there is an effect that it is possible to improve the quality of the upward communication of the PON such as communication state monitoring and presence or absence of the burst light signal. Therefore, it is necessary to detect the light reception level of each of the burst light signals. For example, Patent Literature 1 and Patent Literature 2 propose a technology for detecting a light reception level in a point-to-point system.